1. Field of the Invention
The present invention relates to the manufacture of a compound semiconductor single crystal and, more particularly, to a method and apparatus for manufacturing a compound semiconductor single crystal capable of growing a single crystal having a uniform carrier concentration.
2. Description of the Related Art
In recent years, compound semiconductor single crystals such as GaAs, GaP, InP, and the like are used as substrate materials of various semiconductor devices. These single crystals are manufactured by the Czochralski method, the Bridgman method, and the like. In these methods, if a semiconductor single crystal is manufactured by growing a single crystal from a raw melt, oxygen, carbon, and the like remain in the resultant single crystal as residual impurities. These impurities are produced from graphite normally used as a material for a heater or other furnace members, and are mixed in the raw melt through an atmosphere.
Oxygen, carbon, and the like as the residual impurities in the single crystal normally exhibit a change in concentration along the growth direction of the single crystal, and the change in concentration adversely influences electrical characteristics of a semiconductor single crystal and causes the following disadvantages.
First, desired electrical characteristics are obtained by doping a certain impurity in the single crystal. The electrical characteristics are influenced by the change in concentration of the residual impurity along the growth direction of the single crystal, as described above. Therefore, it is difficult to control the electrical characteristics to be uniform throughout one single crystal ingot. Therefore, the desired electrical characteristics can be obtained from only a limited portion of the single crystal ingot, resulting in very low product yield.
Second, when a semi-insulating GaAs single crystal is manufactured by a liquid encapsulated Czochralski method (LEC method), a carbon concentration in the resultant single crystal ingot tends to be high in a head portion of the ingot and be lowered toward the tail portion. Therefore, when a semiconductor device is manufactured using a substrate prepared from the head portion of the ingot having a large carbon content, the resistance of the substrate is decreased during a heat-treatment process, and a desired semiconductor device cannot be obtained. If the carbon concentration is too low, a semi-insulating property is often already lost and a resistance becomes low when the single crystal is manufactured.
A GaAs single crystal of the compound semiconductor single crystals is widely used as a substrate material for a variety of semiconductor devices, such as a light-emitting device, e.g., a light-emitting diode, a semiconductor laser or the like, a high frequency field effect transistor, a Hall device, and the like. When the GaAs single crystal is used as the substrate material of the light-emitting device, a high-concentration n-type GaAs single crystal is normally used. Recently, a technique of fabricating a light-emitting device on a p-type GaAs single crystal substrate of a high carrier concentration (10.sup.17 to 10.sup.20 /cm.sup.3) has been developed, and a p-type GaAs single crystal has also become popular. As an impurity for the p-type GaAs single crystal, zinc is normally used. However, since the zinc is solid, it must be doped in a raw melt. In addition, since the zinc has a segregation coefficient of about 0.3 with respect to GaAs, the carrier concentration of the head portion of the growth of the single crystal ingot is ten times or more of that of the tall portion. Since the boiling point of the zinc is as low as 906.degree. C., it is very difficult to dope the zinc with high controllability.
In a field effect transistor, element isolation becomes difficult to achieve along with further micropatterning of devices. For this reason, a method of producing a device using a p-type GaAs single crystal substrate of a low carrier concentration (10.sup.14 to 10.sup.17 /cm.sup.3) is proposed. However, the substrate for the field effect transistor has a very strict specification as compared to that of a light-emitting device. As described above, a zinc-doped p-type GaAs single crystal has low reproducibility of a carrier concentration among crystal ingots, and a carrier concentration is also largely varied in one crystal ingot due to its segregation coefficient. For this reason, it is very difficult to obtain a p-type GaAs single crystal for a substrate of the field effect transistor.
As described above, a p-type GaAs single crystal with a controlled carrier concentration is demanded. However, as long as zinc is used as a dopant, it is very difficult to control a carrier concentration to be uniform not only among a plurality of crystal ingots but even in one crystal ingot.
In consideration of the above situation, the present inventors examined various impurities as a p-type impurity in to replace zinc. It has been found that carbon can be used as a p-type impurity, and that carbon can be doped in a gas state, e.g., CO or CO.sub.2.
If an apparatus as a combination of a conventional GaAs single crystal pull device and a carbon supplying system is used, a p-type GaAs single crystal can be obtained. However, the carbon concentration of the resultant single crystal cannot be controlled to a desired value. Furthermore, even if a supply amount of a gas as a carbon source is controlled to be constant, it is difficult to maintain the carrier concentration of the resultant GaAs single crystal to be constant.